linux-kernel - Re: [tip:core/rseq] rseq/selftests/x86: Work around bogus gcc-8 optimisation

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Message-ID: <1950499847.215.1555708401414.JavaMail.zimbra@efficios.com>
Date:   Fri, 19 Apr 2019 17:13:21 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Lameter <cl@...ux.com>, shuah <shuah@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Dave Watson <davejwatson@...com>,
        Boqun Feng <boqun.feng@...il.com>,
        Russell King <linux@....linux.org.uk>,
        Paul Turner <pjt@...gle.com>, Ben Maurer <bmaurer@...com>,
        Joel Fernandes <joelaf@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rseq] rseq/selftests/x86: Work around bogus gcc-8
 optimisation

----- On Apr 19, 2019, at 2:53 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Fri, Apr 19, 2019 at 11:46 AM tip-bot for Mathieu Desnoyers
> <tipbot@...or.com> wrote:
>>
>> rseq/selftests/x86: Work around bogus gcc-8 optimisation
>>
>> At least the following versions of gcc-8:
>>
>>  - gcc version 8.0.1 20180414 (experimental) [trunk revision 259383] (Ubuntu
>>  8-20180414-1ubuntu2)
>>  - gcc 8.2.0-7ubuntu1 (Ubuntu 18.10 (Cosmic)),
>>
>> generate broken assembler with asm goto that have a thread-local storage
>> "m" input operand on both x86-32 and x86-64. For instance:
> 
> Is there a gcc bugzilla for this? Shouldn't that be mentioned here?

My google-fu did not find any match, so I don't think so. I'll open one
shortly.

> 
> Also, we use "asm goto" together with "m" all the time in the kernel.
> In fact, it's the most common case, with the RMWcc ops being generated
> with that. I realize that we don't use the gcc thread-local storage
> for them (we often do use our *own* thread-local storage), but it
> would be good to have that gcc bugzilla to see why it can only affect
> those user level "__thread" cases.

Indeed, it would be good to investigate and understand all problematic 
scenarios triggering this bug.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com